Auto rack cars typically have two or three decks for carrying automobiles to maximize the number of automobiles being transported. As a result of the different designs for auto rack cars, including the number of decks, the heights of the upper decks relative to the lower deck may vary between designs.
Circus loading of the automobiles has long been employed as an efficient method of loading the automobiles onto the cars. In this method, the end doors of each of the cars are opened and bridge plates are connected to the upper decks. These bridge plates span between adjacent cars, thereby allowing automobiles to be quickly and easily driven through numerous cars during loading.
If the upper decks of adjacent cars are at different heights, it becomes difficult to securely connect the bridge plates to the decks. Moreover, as the height differential increases, the angle of the bridge plates also increases. Because of the short length of the bridge plates, even a small differential in deck heights may result in a steeply inclined bridge plate. It thus can become very difficult to drive the automobiles from one car to the next over the bridge plates.